Science in Science Fiction: seasons

The concept of a year divided into four equal parts–spring, summer, autumn and winter–is a northern European one, and the images that go with these seasons are even more so. The northern European seasons are determined by the position of the sun in the sky, and therefore the temperature.

(just to be clear, the seasons are determined by the inclination of the earth’s axis to the plane of rotation and have very little to do with the distance from the sun–more about that later)

Even on Earth, there are many places where the seasons don’t conform to this four-part structure at all. Anyone who has lived on or near the equator can tell you that the four seasons are total rubbish. The “summer” is isn’t any less warm than “winter”, and trees lose their leaves either not at all, whenever they damn well please, or in response to a dry period. Even further away from the equator, seasonal rainfall is a much more dominant factor in determining the season. Before the European concept of spring, summer, autumn and winter was adopted (read: enforced) in these regions, native peoples had much better ways of describing local seasons. They usually involved the absence or presence of rain.

If you live on the equator or anywhere 23 degrees north or south of it, the sun passes over twice a year. The closer you are to the equator, the more equal these parts of the year are. When we lived in Townsville (19 degrees South), the sun was usually in the north except for about three weeks in mid-summer during which it blasted down on the side of the house that never received any sunlight in the rest of the year (hint: my poor plants were usually not impressed by this state of affairs).

The passing over of the sun coincides with a downward air stream, a humid zone, where there is a lot of rainfall. The air cells on either side become much drier until you hit the tropics of capricorn and cancer, where the air stream is upwards. This is where the earth’s deserts are. Another, similar, dry-wet band lies to the north, but here the effect of temperature takes over as season-defining factor. These bands of air volumes where weather–and humidity–circulates are called Hadley cells, and all planets with atmosphere have them. Mars does. Titan does. Venus does. How many cells there are depends on the speed of the planet’s rotation speed and its overall temperature. Titan is cold and doesn’t rotate very fast, so there is only one air cell.

Earth’s orbit is relatively circular, but it is still closer to the sun during the southern hemisphere summer. Closer to the sun means that earth moves faster during that period. So when looking only at the influence of the sun, the southern hemisphere summers are shorter and hotter than the northern hemisphere ones, and the winters on the southern hemisphere are longer and colder. Because this is such a small effect, it gets lost in other influences, such as the mitigating factor that the southern hemisphere is mostly ocean.

But imagine if the orbit was much more noticeably elliptical, like Mars.

So what does all this have to do with SFF writing?

Well, unless your planet’s axis of rotation is perfectly at right angles with the plane of movement around its sun, there will be seasons. Venus is an example of a planet without seasons. The size of the planet and its rotation speed will influence the strength and number of air cells. Anywhere close to the planet’s equator, these cells will have a much greater influence on seasons than the position of the sun in the sky. If you place a colony on a planet, the colony is likely to be on the equator, because it’s easier to launch ships.

In fantasy, a warm climate (anywhere more tropical than mediterranean) means that most likely your seasons will be controlled by rainfall. If your planet is large, warmer and/or has a shorter day than Earth, it means that the weather systems will be much more violent. Think massive cyclones.

If your planet orbits two suns, it is highly likely that its orbit will be very elliptical, as well as much wider than for a single sun. So your year will be much longer, and your seasons won’t fall into neat spring-summer-autumn-winter divisions at all.